Video signal control system



@H6275 1944- L. F. MAYLE VIDEO SIGNAL CONTROL SYSTEM Filed Sept. 19,1942 3 Sheets-Sheet lI2 R ma Nl.- EVI VA NM F.

June 27, 1944;` L. F. MAYL I yVIDEO SIGNAL CONTROL SYSTEM Filed sept.19, 1942 3 sheets-sheet 3 Y E N R O W A INVENTOR MAYLE K Patented June27, 1944 UNITED STATES 1 12,352,488 VIDE() SIGNAL CONfrrtoL sYsfrEMLouis Fl Mayle, Fort Wayne, Ind., assigner to tf t Farnsworth Televisionand'RadionCorporation, j

a 4corporationoftl )e laware ,Y

^ Application september 19, 194g', lSarita 1\Io. "4 5s, 92 V' Thisinvention pertains generally `to television systems and moreVparticularly to amethod and apparatus for controlling and adjusting thel gamma. of television signals.

It is known in the art that the quality of'a reproduced televisionimage'may be impaired as a result of the factthatdetail effects such asare given toapicture by its natural color, are lost. Alsol televisiontransmitting apparatus may be of Asuch a nature that video signalsaredistorted and decreased in level to such an extent that thereproduced signal does not create a faithfulv image of the initialpicture subject matter. v'Furthermorepv each element of a reproducedtelevision image' may be exactly proportional'in brightness to thecorresponding element of the original image and, therefore' thereproduced frequency s i'gnalwit'hout' affectingthne' overall magnitudeofthat signal.

A stm further, object ofthisfinvetiorifi to provide, in a televisionsystem; a signal trans'- lating channeljfor controlling they-anima "fof,a

video frequency signal andirnpartingoyerall v'gain to that signal'. Y`There-is provided, in accordancewith this 'in'- vention, a signaltranslating'channel in atelcvision system whichconsists'ofjamulti-electrodev electronic device for controlling the'garnma'fof avideo frequency signal. 4The'electronic'device image 'may appear' fiatand lack efficient contrast to be interestingjto an observer. Thiseffect may be the result of4 the .fact that the contrast ratio andlaverage brightness of the reproduced -image is generally lower incomparison to the corresponding characteristics of the picture beingtransmitted Therefore, it is frequently desirable in televisiontransmission that videol frequency signals be distorted in certainportions of their amp1itude range for providing better fidelity ofreproduction.'V 'Such distortion is known as gamma control, moreparticularly delned 4as the slope of the 'input-output or stimulusresponse curve of the system plotted'to a logarithmic scale. "Thereareknown in the prior art, various devices for controlling thegamm-a ofvideo frequency signals to provide faithful reproduction cfjtele'visionimages whereby the contrast between the light `and dark portions of atelevision image 'is increased. These devices are subject to 7 Anotherobject of this invention is to provide',`

in a television system, apparatus for imparting a gamma to a videofrequency signal which-may be either greater or less than unity.

It is a further object of this invention to provide, in a televisionsystem, a video signal translating'device for varying the ygamma of avideo should preferably comprise at least ftwoctlfol Y electrodes and ananodafone' control electrode having an input voltage-anode currentcharacter'- istic which tends to provide a gamma greater unity, theothercbntrol "electrode havingfan in'- put voltage-anode'Ycurrentcharacteristic which tends 'to provide a gamma lessthan unity.'The gamma controlchan'ne'l is connected inthe video f frequencyamplifier circuit' Aby means of a voltagedividing device Isuch as' apair of oppositely connected potentiometers, oneipotentiometer' beingconnected'thrllfh its adjustable tanto one'control electrode of theelectronic device and` f ,th,e other potentiometer beingconnectedthroughtits adjustable tap' ,to the other 'control electrodefofthe electronic jdevice ther ebyto provide afmeanfs for dividing a videosignaland lapplying'fa.por-

tion thereof to each controlelectrcde jof`theele`cftronic device. .The`adjustable taps 'on the oppositely connected potentiometers are`preferably 1 control which may be adjusted tofin lout in theappendedyclaims mechanically ,connected tepnrorid" cr'easethe voltage'dropacross onepotentioxneter'and simultaneou'sly decrease the potentialacrossjthe other potentiometer. Thus, 'thelportion of video frequencysignal impressed on each control 'electrode may be varied 'to exactly.apportio'n the video frequency'signalbetweenfthe two controlelectrodes, Since the` characteristic 'of Ione,` control electrode has agamma or exponent. greaterj tha n unity and the characteristic ofthe'other. electrode has a gamma or exponent less than unity, eachportion of the videofeguencylsignal may have im-` partedv thereto algamma either greatcrfofr less than unity; l

For a better 1'"lnderstandingr of" the 'invention together withl further.objects and radvantages thereof,' reference is Imade yto the'following'description taken in connection with the accompanying drawings, and'its'sc'ope will be 'pointed In'the'drawings: rf

Fig. `1 ,isa' circuit diagram of a complete tele.

l Common' vision transmitting system illustrating conventionalportionsof the system in schematic form andincluding avideo frequencyvsignaltranslating channel' embodying the present invention. `Fig. 2 isal graph; illustrating the grid voltage plate current characteristics ofthe control electrodes of the signal translating channel'.

Fig. 3 is a graph, illustrating the operation,

under certair'i-v conditions', of thesign'al translat-` ing channelshown inlFig 1I,

Fig- 4 S a gaph, illustratingthe operation,

under certain conditions, ofthe signal translating channel, shown inFig; l.

1 The .sliding contacts'31y and 381, of potentiometers Referring to Fig.1 of` the drawings, the'tele- .vision transmitting system includesarconventional picture pickup tube I; anf objecta'nd` a.`

lens system 3 for focussing an.;image on the" screen 5 o f tube I'. Thetubev I is provided with electron gun 1 and a power supply i2 to providean electron beam for scanningscreen-fS.' This f tube is illustratedbyway of example, as conventionsiim' the television art, andy it is notfintended that this invention shall bejl'imitedltouse with anyparticular type of pickup device; l v

`AThe' cathode" ray'b'eamipf tube tisdeflected to a source of` linescanning waves and'eld scanning waves I3-and Il respectively. `Thesources t3 and'; tifare. controlled" by. the synchronizingpulselgnerator 'I6 in 'fa mannerwell known in the' prior art 'Ihe outputQ`f screen 5in tube I `is irn'pr'es'sed across the output resistor Vi8connected to"V the in-put circuitof. the picture signal y. .'I l"li ereis provided necte'dfto the Outputcircuit- 2210i picture'signalampliiler' 2l and the output circuit 26 of channel is connected to `alconventional radiant'energy transmitter 3`0`wh`eein the video frequencysignal v is impressed 'on a carrier and radiated vby antenriajli; Q

Signal translating channel 2 5 consists of `the multielectrodeelectronic device 32. and a voltage dividing device consisting ofpotentiome ers 3l and 35 mechanically connected in tandem. Suchaconnectionfis, wellknow'n tb those skilled in the artand is, therefore,not shown in detail. Electronic device 32 is chosen to have thegridvoltageanode Acurrentv characteristics shown in Fig. '2, Curve' A, 'the'grid voltage-anodecurrent `characteristic of grid lfhasan exponent o rgamma less 'than"unity Curve B, the" grid voltage-anodecurrentcharacteristic of grid 4I, has an exponent or. gamrnaggreaterthanunity. For ideal gamma controlin accordance with this invention,curves and`fBfof Eig. 2 'should be complementary, but satisfactory gammacontrol is obtainable eventhou'gh these curves are not exactlycomplementary," Curves A and .B are substantiallyl parabolic in forrfnand, therefore,

follow the'equation X41/ where exponent a 'is the gamma. i l

The `grid voltage-anode current vcharacteristics fof Agrids 40 and Ilmay be illustrated also-on a log-log scale by converting theequationyf-#Xa to its logarithmic formwhich is .log y=a log :c andplottingalogarithmic graph. Curves Aand B would then be vst'raigh 'tllines the slopes of which would be determined by the gamrnaor. term a inthc logarithmic equationi, If gamma is chosen plliler'ZD, there Vbeing aconnection 2ly pi'oatnland pictuefsignal amplifier for" impressingablanking pulse onainplier 20 as is 'fwellknownintli prior art. 'naccordance'with this in "ventiqn, a' signal tranSIatingchannel 25 con 4cated at 39, they may be adjusted simultaneously 34 and 35 respectively; are connected to the control electrodes 40 and`v Hrespectively, of tube v 32. Sliding contacts 3T andii are mechanicallyconnected", as indicated at 39, to provide a coml'x'non control forpctentiometers 34 andl 35, suchy vthat each sliding .contact may besimultaneously moved in the same direction. Control electrodes and 4lvare biased through grid resistors 44 and respectively in `accordancewith the'potential drops as determined by the adjustments of the tapsoflbias resistor 46' which isfconnected toa f source of biasingpotential, C-

The anode 42 ofy tube 32 is connected to transmitter 3U and to a sourceof anode potential 'B+ togetherwith a peaking circuit consisting ofresistor` 4 6 and inductance 41 which is conventional in the prior art.lThe remainder of the signal translating channel consists of well knowncircuit elements and connections and. therefore, it is not necessarythat they be explained in detail. It should 'be noted that signaltranslating'channel 25 may by a cathode ray beam tc impress a videoire-y quency vsignal consisting of a.chain of electricalir'npulsesfacross the resistor I8 representative of the successivelyscanned elemental areas 'of -thc object 2. The video frequency signalappearng across vresis tor. -l"!l is amplified in a. conventional mannerby picture signal amplier 2D and impressed on the output circuit 22.

Signal translating channel 25 imparts a gamma to the picturesignaleither greater or less than unity dependingfupon the adjustment ofthe potentiometers 34 and 35, and upon` the gamma of the video signaldelivered by the picture pickup device. Sincethe adjustable slidingcontacts 31 and 38 areimechanically connected as indito `move upwardlyor downwardly, increasing,rr the output oi one from zero 4to maximuinandde-.

creasing the output of theotlier from maximum to zero. Thus. forpurposes of illustration, it may be assumed that the mechanicalconnection simultaneously moves sliding contacts 3 1 and 38 upwardly o'rdownwardly. so that the signal appeering in the outputcircuit 22 maybedivided in any desired proportions between the Acontrol electrodes 4'0and 4I.

Fig. 3 illustrates. by means of agrapli, the resultswhen the voltageacross the output circuit 2 2 is impressed on the grid 4 0or grid 4I `of-tube 32. It also illustrates the 'results when one-.half ofthe voltageacross output circuit 22 is impressed on each of. the grids 4U and 4|;Curve c in Fig. 3 represents the fu-ll video voltageY across outputcircuit22. For purposes ofzexample, curve.

:on `grid 54,0.

'.e represents a Avideofvoltage `wavefmoving eight equal: incrementsofvbrillianceV from black vto gridA M, the .voltage resulting l in `theoutput 'of tube2,3 isrepresented by curveC.` Curve .B is assumed tohave. an exponent` or gamma greater than'unity, jFor'purpose'sofzzexample, curve B- has anzexponent of 2. Thus, vthe Avideoou'tput at`2.2' has had lits, gamma inci'easedby a factor-of 2.

Since 'curve ehadl'a gamma. of"1, curvellC hasfa gamma of ,2.-From'inspection of curve Citjis ap,-

par'entfthatthe lblaclfrfportion of the signal is `expanded .whilethewhite portion is contracted.

, Thus :black'portions Yofth'e transmitted' picture areiemphasized withrespect to its white portions.'

" 'It the potentiometer settingis `changed ysotliat both sliders 3landf38 are moved upwardly to the fuppe-r limit of 'theirmovementpfulllvideo voltage e-,is impressed on theigridll' andthereiszero videoyoltage-ongrid M.l Again referring' to Fig.

3., there `is shown a projectionof curve e] to curve A," which is Athecharacteristic of grid 40, and voltkageHlappears Vin the output of tube32,?tbi`svoltage having-a gammacorrespondin'g to `Vthatof curve VAwhich'is Afory this-example', equalto 1/2. Inspectionof curve AH'reveals that the white por-V tion ofthe signalisgexpanded and the black-por tion1 is contracted thereby 'to' emphasize v,the white 'portions ofthe'transmitted picturewith'respect i to lits blackportions. ".'Inthiscase the -gaminaiof curve'ie Yhas,beenrchanged fromi to 1/2 as `repre-By adjusting V the Sliders ai-andxss ef the petentiometer to thehalf-way position, one-'halfbf 'the `voltage e-is impressed ion eachofthegi'ids-U `vanddl. Thereisillustratedin Fig.3 by the curve 1/'e..`Projecting this ,curve to ytliefcurves A `and Bthereresults the` curves1D andD respectively.

curve' D having' a gamma ,of k2". By` adding these two curves, there isobtained the output voltage which appears in the lanode circuit,oftube`32. This." s=illust-ratedY by thecurve D-i-.D `whi'ch -has agamma of approximately one since it lies in substantially a straightline. In this case there is substantially no distortion of signal.'Thus, the transmitted picture is not distorted as regards contrastbetween black and white.

Fig. 4 of the drawings illustrates the results when the potentiometer isadjusted to impress 3A of the output voltage in circuit 22 on one gridand 1A of the output voltage on the other grid. In

.the first instance, let it be assumed that the po-l tentiometer isadjusted downwardly to the point --where 135, of the output voltage incircuit22 is -a curve F-l-F' having a gamma greater thanunity andexpanding in amplitude the black portion of the signal, and contractingthe white portion, but

not as much as represented by curve C of Fig. 3.

Reversing the above procedure, the potentiom- Y eter is adjustedupwardly until the sliders'31'and 38 impress 1A; e. e being the outputvoltage in circuit `22, o n grid 4| and %Je ongrid 40. By

quencylsisnal.

I acter-istics'such asfthose 'illustrated l these illustrated iuris; mayutilized' fi .s

. illustrated herein.

7 noty haveja gammagreater jecting the voltage l/l;l ecn vrnirye-Bthereis tained? a voltage componentG. By'a'dding'cu es- ThelcurvSAfald B of 'Figs i, to have'an'rbitrary exponent orgflma'forpurposes' of fillu,stration,f.;but it willbey obvious"tOi-'those ,.suchf'rves isy .dependent MP6- he ,grid ,Chre ,eristics ofthefpar'ticular 'tube edjin thejsignal 'translating lhannel;z It ha`...blee fun'd that tube conventionally desifg'natedfas 'a' 'SLThas c,

One ohara'cteristichaving a gamrnai'gr ter. than unityand the othercharacterist j h'avin`g` a ygi-,im' ma lessthan unity.v Anymultieelectrode v zzu tube whose, grid characteristics are similar-tol`signal translatingl channel4 for' the purposes complishin'gf the.objects of" this invention therefore', it ie 'not intended 'that shallbe'lifmited tothe particu It should ybeunderstood;thatdifferent of'gamma control may'ibe 4intredueedingef television systemtocompensatedistortion ofa video 4 frequencyl signal which' may result.;for example, from the 'particular type 'of' pictur'elpiclgup 'tubewhich` isused,y orffrorn the particular-transmit,- ting orreceiving"channel.l Therciirves'A'a'n such Aas i1111stratedf '.Fig'. 2.'o`f` thedrawings, ned ty'v less than' un'ity by `'reason' ofthefactjthat' distoif tions of "theisgnalA may require that one curve. havealgam'maof. unitvfand the othe'r'have gamma" g'reater thany unity.' AItlmay be 'fdesirable that both curves have agamma greater; than'unityforless than unity or it'may bdcslrable that a portion of eachcurve have la gamma of unity and another portion have a gam-ma greateror less than unity. From this it -will be obvious to lthose skilled inthe art that it is within th'e scope of thisinvention to provide'agammaad.-

justing channel for distorting a video frequency signal to any degreewhich 'may benecessary to bv one or moreeleent considered the'preferredembodiment of the invention, it will'l be obvious to those skilled inthe art,.th'at various .changes and modifications 'l may befmade.therein without departing from the' invention," and 'it' is, therefore,aimed in the lap.-

pended claims tocover all such changes and modi.-

skilled `infthe art-that `the'shapeorexponent".of

a .circuit oi those 1.*,Atelevision signal translating systemfcomnprisinga signal channel, a Viirstimeansl for con- -vtrollinga ow ofelectrons, a second means `for controlling a owwofelectronatheinput'voltageanode current `characteristic of said first means providinga gamma greater than unitythe input voltage anode current characteristicof said sec-4 ondmeansproviding a gamma less than unity, 'Y fandervoltage ,dividing means coupled-between said channelz-and said rstandsecondineansfor dividing the television signal andimpressing it on `eachoffsaid first .and 'second means-,to-'impart a Igamma to theresulting`signalwhich Vmay be either greater or less than unity.

j 2'.` Av television .Y signaltranslating usystem compriising a y signalchannel, `a `inulti-(electrodei electronic-deinem the input yvoltageanode current `charanteristic of one electrodeprovidingfagamma greaterthan` unity, the input, voltage-anode. current characteristic of anotherelectrode providing fa gamma lesszth'an unity, and a- .voltage dividingfmeans, coupled betweenk said channel and said electrodes forsimultaneously dividing the teleyisiorr signal and impressingv iton eachof said electrodes toinparta gamma tothe resulting signal whichmayeither greater or less than 3. television signaltransiatingsystemgcom- "lprising a signal channel, a linulti-,electrode,electronic device, the input voltage-,anodecurrent anode currentcharacteristic of another electrode providinga gamma lessvthan unityya'lfst outn y 12,352,488 4 ilcations asl fall within thetruespiritandvscope tronic n device,y 4the input `voltage-anode current,ofthe'invention., Y v l l characteristic clone grid providing a gamma lWhat I claim is: greater than unity, the input voltage-anodecurrentvcharacteristic of another grid providing a gamma less thanunity," a first kinput poten- ,tiometer coupled between said channel'andsaid onegrid, a second input potentiometer coupled between-saidchanneland said other grid and control means. l common l to saidy.potentiometers n the television signal-insaid channel mayfbe dividedand impressed on eachl of said grids to impart. afgammato the resultingsignal which may be either greater or4 less .than unity.,V

5. I'I'he methodof translating atelevision signal comprising the stepsofi-dividing a kvideo signal intoJat least two components, impartingagamma,

greater` than unity to one of said components. .imparting a gammalessthan unity tothe other vof; said components and combining saidcom-po- Ynents to obtain a signal having a gamma either sum of which isequalto said given voltage, im-

putpotentio'meter coupled between said ,channel andsaidV one electrode,a 'seond inputpotentiorneter'coupledbetween'said channel and said `otherelectrode and control meanssforfoperatingsaid potentiometerstoincreasevthe output of one potentiometer and decreaseftheoutput Vofthe. 'othery potentiometer vwhereby the television "signal may bedivided and. impressed v[on `eachol said yelectrodes to. impart a gammato the resulting signal which vthan unity.

prlslnga television channel, a multi-grid 'elecparting a gamma greaterthan unity to one of said components, imparting a gamma less than unityto -the other ofl said components and combining-saidlcomponents toobtain a signal having aV gammaeither greater or less than unity and anamplitude substantially equivalent. to the amplitude of said videosignal.

'7. The method of translating a television signal comprising the stepsof dividing'a' video signal of a given voltage into voltagecomponentsthe sumr of which ls equal to said given voltage,

may begveither greater or less v4. Ateievisin signal trailsiatingsystmomlimpressing each component on `a different gridof anzzreletron tube,imparting, a lgamma greater than-,unity to one oi." said components,impartinga gamma'less-than unity to the other ofsaid componentsand';controlling,the output o! said electron ,tube 'by said componentsto obtain a ,signa-l having a gamma either greater or less than unityand a magnitude substantially equivaient -to the magnitude of the inputsignal.

